Long Term Evolution (LTE) specifications from the 3rd Generation Partnership Project (3GPP) support component carrier bandwidth up to 20 MHz. However, in order to meet the International Mobile Telecommunications Advanced (IMT-Advanced) requirements for (very) high data rates, the concept of carrier aggregation has been introduced to support bandwidths larger than 20 MHz. The carrier aggregation concept is illustrated in FIG. 1, where five component carriers, or cells, are illustrated, with 20 MHz of bandwidth each. In the example of FIG. 1, the total bandwidth available to a mobile terminal is the sum of the bandwidths of the cells, i.e. 100 MHz.
Note that in the context of carrier aggregation, a component carrier also refers to a cell. Hence five components carriers as illustrated in FIG. 1 correspond to five cells.
A terminal or a user equipment (UE) may be configured with a subset of the cells offered by the network and the number of aggregated cells configured for one terminal or UE may change dynamically through time based on for example terminal traffic demand, type of service used by the terminal, system load etc. A cell which a terminal is configured to use is referred to as a serving cell for that terminal. A terminal has one primary serving cell (called PCell) and zero or more secondary serving cells (SCells), the term serving cell includes both the PCell and SCells. Which cell is a terminal's PCell is terminal-specific. The PCell is considered more important and for example some control signaling is handled via the PCell. Hence in case of five components carriers as shown in FIG. 1, the terminal may have one PCell and zero, one, two, three or four SCells. As mentioned some control signalling is handled via the PCell and therefore the PCell is an important carrier for the terminal.
Aside from that the concept of configuration of cells/carriers has been introduced the concept of activation has been introduced for SCells (not for the PCell). Cells may be configured (or deconfigured) using Radio Resource Control (RRC) signaling, which can be slow, and at least SCells can be activated (or deactivated) using a Medium Access Control (MAC) control element, which is much faster. Since the activation process is based on MAC control elements—which are much faster than RRC signaling—an activation/de-activation process can quickly adjust the number of activated cells to match the number that are required to fulfil data rate needed at any given time. Activation therefore provides the possibility to keep multiple cells configured for activation on an as-needed basis.
When a terminal or UE gets configured with a cell it may need to re-tune the radio frontend (RF) to cover the spectrum of the configured cell and to change the carrier frequency. Similarly, when a serving cell is deconfigured the terminal may need to re-tune the radio frontend so as to not cover the deconfigured cell. As a consequence of radio frontend re-tuning the terminal may need to perform an interruption, or glitch, during which the terminal is not able to receive of transmit signals using that radio frontend. An example is shown in FIG. 2 and FIG. 3. In the example of FIG. 2 the terminal is configured with Cell A and Cell B but not Cell C. This is indicated by “covered spectrum”.
In FIG. 3, the terminal is configured with all 3 cells A, B and C. When also Cell C is configured the terminal may need to perform a radio frontend re-tuning and hence perform a glitch or interruption. Similarly with deconfiguration, if the terminal cell configuration is first as in FIG. 3 but at a later stage Cell C is deconfigured the terminal may retune the radio frontend to enter the configuration as in FIG. 2.
When a cell/carrier is activated or deactivated the terminal may also perform a glitch, similar to the case of configuration or deconfiguration.
Hence, in order for a terminal to be able to use a cell for transmission, the cell first needs to be configured for the terminal. Cell configuration may be handled on RRC level and the network or the network node is configured to send to the terminal a RRC message ordering the configuration of the cell. The terminal is required to execute the order within a delay referred to as RRC processing delay, which currently is specified to be 20 ms in case for cell configuration. After the 20 ms has passed and the terminal has performed the cell configuration, the terminal responds to the network or network node by sending an RRC message indicating that the configuration is complete.
When the network or network node has received the RRC message from the terminal indicating that the cell configuration is complete the network (node) can send to the terminal an order for activation of the cell. The terminal is required to execute the order for activation within a time period e.g. 8 ms, after which the terminal is configured to respond to the network node with an acknowledgement of the activation command. This delay is referred to as activation delay in this disclosure.
It should be noted that in case the new cell has an uplink configured and the terminal has no valid TA (Timing Advance) value for the new cell the terminal may need to perform a random access procedure to get a valid TA value for the new cell.
So the total time it takes to get a currently not configured cell ready for communication for a terminal is:(Scheduling delay of cell configuration message)+(RRC processing delay)+(Scheduling delay for activation order message)+(activation delay)+(Scheduling delay for random access procedure order message)+(time for random access procedure)
If this total time is long it will negatively affect the user experience and network performance.
It is currently discussed in 3GPP if it is necessary to extend the activation delay. However, an extension of the activation delay leads to that the total time above becomes longer. The reason for doing so would be that the terminal may need to perform a glitch, or interruption, due to RF tuning upon cell activation during which the terminal may not be able to communicate with the network on some, or all, cells. So not only would the delay for enabling communication on a cell be extended but also it could possibly effect the communication between the terminal and the network on other cells/carriers. This would further degrade user experience and the network performance.